FIG. 1 shows a well-known three phase power inverter 100 for converting a DC power supply 101 to an AC output 103. The inverter comprises three separate phases 200, 300, 400. Each phase includes two switches in series: 200a, 200b in phase 200; 300a, 300b in phase 300; and 400a, 400b in phase 400. Switches 200a, 300a and 400a are connected to the positive rail 105 and switches 200b, 300b and 400b are connected to the negative rail 107. In FIG. 1, each switch is an IGBT (insulated gate bipolar transistor). However, any switches with fast switching capability may be used.
A sinusoidal output current can be created at AC output 103 by a combination of switching states of the six switches. However, the inverter 100 must be controlled so that the two switches in the same phase are never switched on at the same time, so that the DC supply 101 is not shorted out. Thus, if 200a is on, 200b must be off and vice versa; if 300a is on, 300b must be off and vice versa; and if 400a is on, 400b must be off and vice versa. This results in eight possible switching vectors for the inverter, as shown in Table 1. In Table 1, the vector values are the states of the three upper switches 200a, 300a, 400a, with the three lower switches 200b, 300b, 400b necessarily taking the opposite state to avoid shorting out the DC supply.
TABLE 1Vector200a300a400a200b300b400bV200-300V300-400V200-400V0 = {000}OFFOFFOFFONONON000ZeroV1 = {100}ONOFFOFFOFFONON+Vdc0−VdcActiveV2 = {110}ONONOFFOFFOFFON0+Vdc−VdcActiveV3 = {010}OFFONOFFONOFFON−Vdc+Vdc0ActiveV4 = {011}OFFONONONOFFOFF−Vdc0+VdcActiveV5 = {001}OFFOFFONONONOFF0−Vdc+VdcActiveV6 = {101}ONOFFONOFFONOFF+Vdc−Vdc0ActiveV7 = {111}ONONONOFFOFFOFF000Zero
FIG. 2 shows the six active vectors and the two zero voltage vectors of Table 1 graphically portrayed in an inverter voltage switching hexagon. Vectorial representation of three-phase systems is well known to the skilled person and will not be described in detail. However, in general, any three-phase system can be represented uniquely by a rotating vector VS, as shown in FIG. 2. The rotating vector VS comprises components of the six active vectors shown in Table 1 and FIG. 2. The voltage at the AC output 103 can be changed by varying the ratio between the zero voltage vectors V0 and V7 and the active vector VS (comprising components of V1 to V6) (the modulation index) by pulse width modulation (PWM) techniques.
FIG. 3 shows an example of pulse width space vector modulation over one switching period according to the prior art. The switching function for each switch 200a, 300a, 400a is a time waveform taking the value 1 when the switch is on and 0 when the switch is off. Referring to FIG. 3, during the first period t0/2, all three switches 200a, 300a, 400a are off (value 0) which produces vector V0 of Table 1. V0 is a zero voltage vector, so this time period is an inactive period. In the second period t1, switch 200a takes the value 1 and switches 300a and 400a take the value 0, which produces vector V1, which is an active vector. In the third period t2, switches 200a and 300a take the value 1 and switch 400a takes the value 0, which produces vector V2, which is also an active vector. Finally, during the fourth period t0/2, all three switches 200a, 300a, 400a are on (value 1) which produces zero voltage vector V7 of Table 1. Thus, the active periods are t1 and t2 and the inactive period is t0. The ratio between the total active period (in this case, t1+t2) and total inactive period (in this case, t0/2+t0/2=t0) determines the output voltage at the AC output.
The resulting waveforms at the AC output generally have spectral components at integer multiples of the fundamental frequency. A large proportion of the discernible, and annoying, inverter switching noise is produced by these components of the switching frequency. This issue is of even more concern if there are periodic components in the output voltage. Harmonic components may also create mechanical resonance, which can be problematic in mechanical systems.
Because of this, and other, problems, the control of switching converters is an area of increasing interest. Various techniques have already been employed to tackle the problem of inverter switching noise. For example, the switching period may be varied from one period to the next, either using a sweep, band limited white noise or random weighting. This reduces the peak of the switching frequency fundamental component by spreading the frequencies. However, this method requires a complex PWM modulator and the continual re-scaling of the current measurement or gain, within a closed loop current control system. Another approach is the use of an injected period at a higher and non-multiple switching frequency. In this method, a pattern of periods of a higher switching frequency is swapped for the nominal switching frequency period pattern at intervals of multiples of the nominal switching frequency period pattern. However, a disadvantage of this method is the reduced accuracy of the inverter thermal model which generally runs at a slower cycle than the injection cycle.
It is an object of the invention to provide an improved method and control system for reducing noise in a power converter.